Conventional stents for blood vessel repair are generally hollow and cylindrical in shape and have terminal ends that are perpendicular to the longitudinal axis. In use, such a stent is positioned at the diseased area of the vessel and, after placement, the stent provides an unobstructed pathway for blood flow. Placement of the stent may generally be achieved by using an elastic material for the stent with the stent retained in a curled up configuration inside a sheath such that the stent expands when the sheath is removed. Alternatively, a memory material such as NiTi which expands to a previously defined position on a change in ambient temperature can be used. Probably most common, the stent may be made of a malleable material and in a configuration such that it can be expanded by an angioplasty balloon catheter inside it.
When attempting to repair a lesion at a bifurcation of a blood vessel, the following problem arises. Because the terminal ends of conventional stents are perpendicular to the longitudinal axis, it is only possible to repair a lesion at the bifurcation completely without obstructing the blood flow downstream from the bifurcation if the side branch vessel extends from the main vessel at an angle of 90 degrees. This is unlikely to be the case in anatomical conditions.
In any anatomical bifurcation at which the side branch extends at an angle different from 90 degrees from the main branch, the repair will either be incomplete if one edge of the terminal end of the conventional stent is placed at the ostium in the main vessel or the main vessel obstructed by one edge of the terminal end if the conventional stent is to fully cover the side branch.
Various systems and methods have been proposed in order to overcome the problem outlined above. In a first approach, a conventional stent is advanced through the side branch and placed such that it fully covers the side branch vessel wall and ostium, meaning that a substantial portion of it will protrude into the lumen of the main vessel. In a second step, a balloon is advanced down the main vessel and expanded to crush the protruding stent material against the vessel wall of the main branch adjacent to the ostium. In a third step, a second stent is advanced down the main branch and placed adjacent to the ostium using a balloon which is then backed up and, in a fourth step, is advanced into the side branch and expanded to open up an aperture in the stent corresponding to the ostium. This procedure relies on the main branch vessel wall to be sufficiently stable to allow the protruding end to be crushed against it, which clearly bears a risk of injuring the vessel. Moreover, there is a significant risk of dislodging the atheroma and other material forming the lesion to be repaired which bears a risk of causing a heart attack in the patient.
US2006/0079956 (Eigler et al) shows one example of this approach of crushing a protruding portion of a side branch stent against the main branch vessel wall. A proximal portion of the side branch stent is arranged to be more easily crushable for this purpose.
An alternative known approach relies on two stents with corresponding balloon catheters being advanced down the main branch, whereby the catheter of a trailing one of the stents is threaded through the leading stent. As the assembly approaches the side branch, the catheter of the leading stent is threaded into the side branch and the catheter of the trailing stent is advanced past the ostium remaining in the main branch. Following placement of the leading stent, such that it is partially inside the main branch and partially inside the side branch, the trailing stent is advanced through the leading stent guided by its catheter and is then placed partially inside the leading stent and partially inside the main branch beyond the ostium to, in essence, implement a T-shaped stent arrangement. This approach also has a significant drawback in that two stents need to be sequentially advanced through the main branch, which requires careful manipulation of the two catheters such that each one is placed in the correct branch and in that a double layer of stent material (two stents overlapping each other) remains in the main branch resulting in a greater risk of complications. An example of this two stent approach can be found in US2004/0172126 (Hojeibane).
An alternative stent for repairing a vessel at a bifurcation without obstructing blood flow through the bifurcation is disclosed in U.S. Pat. No. 5,607,444 (Lam). The stent includes a flaring portion intended to cap the ostium of the bifurcation. However, the flaring portion does not fully cover the vessel walls surrounding the ostium and the stent requires a specifically shaped balloon to be placed. Moreover, this may be less suitable for a lesion which extends substantially both within a side branch and a main branch of a bifurcated vessel.
A discussion of known side branch stents and methods can be found in “Bifurcated Stents: giving to Caesar what is Caesar's”, Alexandre Abizaid et al, EuroInterv. 2007; 2:518-525.